Styrene-diene block copolymers have been formulated in the past to produce a number of types of adhesive compositions. The basic patent in this field, Harlan, U.S. Pat. No. 3,239,478, shows combinations of these block copolymers with tackifying resins and paraffinic extending oils to produce a wide spectrum of adhesives. However, four serious limitations of these adhesive compositions are their relatively low service temperatures, poor ultraviolet (UV) resistance, poor solvent resistance and high viscosity. Generally, the highest temperature at which these styrene-diene block copolymers retain useful properties and act like a vulcanized rubber is limited by the softening temperature (Tg) of the styrene end block. Depending upon the molecular weight of the end blocks and the load, these block copolymers can begin to significantly creep or flow at 120.degree.-180.degree. F. For a number of applications it would be very advantageous to have higher service temperatures. For example, these adhesives would be useful in paint shops if the masking tapes produced had solvent resistance and a 225.degree.-250.degree. F. service temperature. They would also be useful as laminating adhesives in, for example, retortable food pouches if they could withstand boiling water temperatures and in furniture laminating adhesives if they could bear moderate loads at 250.degree. F. In addition, although these adhesives can withstand very short contact with common solvents, prolonged contact with aromatic solvents or blends containing aromatic solvents will cause these adhesives to soften and lose cohesive strength. Further, the relatively high viscosity of these prior art styrene-diene block copolymers also limits their applicability ot certain end uses and certain machines for applying the adhesive to a substrate.
One means to improve the service temperature of these styrene-diene block copolymer adhesive compositions is to chemically cure the adhesive with a phenol formaldehyde resin and heat as disclosed in Korpman, U.S. Pat. No. 3,625,751. However, this procedure is undesirable because it is a very energy intensive process to heat the adhesive after it has been applied to the backing. Also, this process cannot be used on heat sensitive backing such as plastics because the backing melts at the temperatures required to cure the adhesive. A more energy efficient process is the radiation initiated cure disclosed in Hendricks, U.S. Pat. No. 2,956,904. Compositions disclosed, however, did not contain a supplemental crosslink promoting ingredient and therefore, as will be shown here, do not give satisfactory results under commercially acceptable conditions.
A new polymer composition that can be easily cured has now been found.